DOT 4 fluid for clutch- why?
 

My avatar truck is an '02 K2500 with the 8.1/ 6spd manual. Noticed the other day the clutch reservoir was on the low side and *then* I noticed the cap said "DOT 4 only". I just had DOT 3 brake fluid on hand so I did a little research and made a phone call to a trusted mechanic- long story short I ended up going to town for a bottle of DOT 4. The brake system in this truck takes DOT 3 and I end up wondering why the clutch wouldn't use the same fluid?

(if this has been covered somewhere, apologies, the search results were inconclusive)

Re: DOT 4 fluid for clutch- why?
 

DOT3 & DOT4 are both glycol based brake fluids. DOT4 has a significantly higher dry and wet boiling point. I'm not sure why they would use it in the clutch and not in the brakes from the factory. Good possibility someone changed the clutch master cylinder reserviour cap.

DOT4 is downward compatible so you don't have to clean the system entirely to update to DOT4. I run DOT4 in everything. An extra couple hundred degrees boiling point is a good thing when towing.

I changed the brakes on my 05 Silverado and my 09 Impala over to DOT4 when I flushed the brakes several years ago. I replaced the DOT3 master cylinder cap with the 92457850 DOT4 MC cap.
This is the post on flushing the brakes in my build thread. http://67-72chevytrucks.com/vboard/s...8&postcount=68

Re: DOT 4 fluid for clutch- why?
 

hatzie: thank you. It never occurred to me the cap might have been swapped out. Also thanks for explaining the difference between 3 and 4. My local guy said the same thing you do, that 4 could be added to 3 but not vice versa, but he wasn't real keen on the idea. The truck spent the first ten years of its life in the southwest US (not sure precise location) and maybe that has something to do with using a fluid with a higher boiling point

Re: DOT 4 fluid for clutch- why?
 

The service manual calls for GM# 88958860

That is DOT 4 fluid. Nobody switched the cap.

Re: DOT 4 fluid for clutch- why?
 

I've never had a T800 with a manual transmission.
Not sure what the reasoning was to do that. The clutch on my T400 used DOT3.

As I said. I run DOT4 in everything with brake fluid in it.

Re: DOT 4 fluid for clutch- why?
 

You're both right, it seems. The owner's manual refers to "GM Hydraulic Clutch Fluid" (which until now I didn't know was DOT-4) with the part number 95 S Trucker listed for the 6 speed, and DOT-3 for the 5 speed. I guess if they had to make things this complicated it's no wonder people gave up on sticks.